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Estimating rotation periods and magnetic cycle lengths of active stars - PhD thesis by Nigul Olspert (Nov 16, 2018)

Nigul's thesis - IIPeg cyclicity
Nigul Olspert, M.Sc., will defend the dissertation
"From periodic to cyclic processes in stellar magnetic activity research: time series analysis methods and their applications"
on Friday 16 November at 12 noon at the Aalto University School of Science, lecture hall AS1, Maarintie 8, Espoo.

In this dissertation we introduce and develop time series analysis methods which are dedicated to period and cycle length estimation of magnetically active stars. Knowing both of these quantities is important as it makes observable reality comparable with predictions from the dynamo theory. Magnetic activity is primarily manifested through dark spots on the surface of the star. However, the rotation of the star is not uniform, but differential, which makes the period estimation not a trivial task. Furthermore, over time the number of spots changes, forming a cyclic process. For instance, for the Sun the activity cycle is known to last approximately 11 years, while both the length of each individual cycle as well as the amplitude is varying. Such a behaviour is called quasi-periodic.

Opponent: Professor Ivan Andronov, Odessa National Maritime University, Ukraine

Kustos: Aki Vehtari, Aalto University School of Science, Department of Computer Science

Supervisors: Maarit Käpylä (Max Planck Institute for Soar System Research) and Jaan Pelt (Tartu Observatory)
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ReSoLVE is hosting Prof. Axel Brandenburg's 60th Birthday Symposium (April 2019)

"Turbulence & magnetic fields - from the early universe to late-type stars"

Turbulence & magnetic fields - from the early universe to late-type stars

April 1st to April 5th 2019. The venue is the magnificent lakeside hotel Gustavelund, lake Tuusula, southern Finland.

Register here

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The Sun shows signatures of turbulent dynamo action

The Sun shows signatures of turbulent dynamo action

We have determined magnetic helicity spectrum from the solar surface observations using the recently developed two-scale formalism. We analyzed synoptic vector magnetograms built with data from the Vector Spectromagnetograph (VSM) instrument on the Synoptic Optical Long-term Investigations of the Sun (SOLIS) telescope during January 2010 - July 2016, hence covering a large fraction of the solar cycle 24. Our study includes the total of 74 synoptic Carrington rotation maps. We recover here bihelical spectra at different phases of solar cycle 24, where the net magnetic helicity in the majority of the data is consistent with a large-scale dynamo with helical turbulence operating in the Sun. More than 20 precent of the analyzed maps, however, show violations of the expected sign rule.
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Surface f-modes indicative of subsurface magnetic fields in the Sun

The Sun shows signatures of turbulent dynamo action

Recent numerical work in helioseismology has shown that a periodically varying subsurface magnetic field leads to a fanning of the f-mode, which emerges from the density jump at the surface. In an attempt to model a more realistic situation, we now modulate this periodic variation with an envelope, giving thus more emphasis on localized bipolar magnetic structures in the middle of the domain. We argue that detections of f-mode perturbations such as those being explored here could be effective tracers of solar magnetic fields below the photos phere before these are directly detectable as visible manifestations in terms of active regions or sunspots.
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